David J. Bottjer

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Aftermath of the Permian-Triassic mass extinction event: Paleoecology of Lower Triassic carbonates in the western USA

Paleoecologic study of invertebrate faunas from three successive Early Triassic seaways reveals that biotic recovery from the end-Permian mass extinction event was slow, and that full recovery did not occur until after the Early Triassic. Simple, cosmopolitan, opportunistic generalists, and low-diversity, low-complexity paleocommunities were characteristic of the entire Early Triassic in the Western USA. An increase in guild and taxonomic diversity was observed with the addition of several new higher taxa in the late Early Triassic (Spathian) to the almost exclusively molluscan faunas of the earlier Early Triassic (Nammalian). Potential “disaster forms” (the inarticulate brachiopod, Lingula, and the paper pecten, Claraia) dominated the earliest Early Triassic faunas (Griesbachian) and even occurred in the late Early Triassic (normal marine stromatolites). Comparison with data on faunas from the Permian and Triassic suggests that even the most diverse Early Triassic faunas (in the Spathian) were rather low in guild diversity and species richness. These characteristics of genera and paleocommunities in the Early Triassic may be typical of mass extinction aftermaths.

Trace-fossil model for reconstruction of paleo-oxygenation in bottom waters

Recognition of fluctuations in the degree of paleo-oxygenation of bottom waters recorded in fine-grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions. General sedimentary fabric, composition of trace-fossil assemblages, and burrow size and crosscutting relationships have been incorporated into a trace-fossil tiering model that permits detailed reconstruction of changes in paleo-oxygenation of bottom waters. Applications of this model to the Miocene Monterey Formation (California) and the Cretaceous Niobrara Formation (Colorado) indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body-fossil information.

Tiering in Suspension-Feeding Communities on Soft Substrata Throughout the Phanerozoic

Tiering of benthic marine suspension-feeding communities on soft substrata has varied throughout the Phanerozoic. Epifaunal tiering was most developed during the middle and late Paleozoic and the Triassic to Jurassic, with large-scale reductions in tiering occurring during the Permian-Triassic extinctions and after the Jurassic. Infaunal tiering reached its highest level of organization after the Paleozoic.

Onshore-offshore patterns in the evolution of phanerozoic shelf communities.

Cluster analysis of Cambrian-Ordovician marine benthic communities and community-trophic analysis of Late Cretaceous shelf faunas indicate that major ecological innovations appeared in nearshore environments and then expanded outward across the shelf at the expense of older community types. This onshoreinnovation, offshore-archaic evolutionary pattern is surprising in light of the generally, higher species turnover rates of offshore clades. This pattern probably results from differential extinction rates of onshore as compared to offshore clades, or from differential origination rates of new ecological associations or evolutionary novelties in nearshore environments.

Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition

Sedimentary structures produced by microbial activity are well known from ancient carbonate facies, but little is known about equivalent microbial structures in ancient siliciclastic facies. Wrinkle marks and Kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics, and they may represent microbial activity. To evaluate this possibility, these structures were studied in Vendian–Cambrian strata of North America and compared to structures formed by modern microbial mat communities in Redfish Bay, Texas. Striking similarities in sedimentologic, petrographic, and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes. The relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the Ordovician, microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea-floor environments.

LOWER TRIASSIC LARGE SEA-FLOOR CARBONATE CEMENTS : THEIR ORIGIN AND A MECHANISM FOR THE PROLONGED BIOTIC RECOVERY FROM THE END-PERMIAN MASS EXTINCTION

Precipitation of inorganic calcium carbonate is a common occurrence in both modern and ancient marine environments. However, synsedimentary growth of large (>5–10 cm) crystalline carbonate cements directly onto the sea floor has been thought to be limited to the Proterozoic, when seawaters were highly oversaturated with calcium carbonate compared to average Phanerozoic values. Outer shelf to slope deposits of the Lower Triassic Union Wash Formation in east-central California, deposited in oxygen-restricted settings, contain crystalline calcium carbonate cements that appear to have grown directly on the sea floor. Paleoenvironmental analyses indicate that these large calcium carbonate cements grew under conditions that were similar to those proposed for the precipitation of inorganic calcite in the Black Sea. Sulfate reduction of organic matter led to an increase in the amount of bicarbonate ion in deep waters and a concomitant increase in ΣCO 2 and alkalinity. Mixing with surface waters led to CO 2 degassing, and precipitation of cements from waters supersaturated with calcium carbonate. The presence of these cements and associated facies thus provides evidence of harsh environmental conditions in the Early Triassic at the regional level, which may have acted in concert with biotic effects of the end-Permian mass extinction, as well as similar deleterious conditions (e.g., shelf anoxia) in other regions, to produce a prolonged as well as temporally and geographically variable biotic recovery from this mass extinction.

Phanerozoic development of tiering in soft substrata suspension-feeding communities

Tiering is the vertical distribution of organisms within the benthic boundary layer. Primary tierers are suspension-feeding organisms with a body or burrow that intersects the seafloor. Secondary tierers are suspension-feeders that maintain positions above or below the sediment-water interface as either epizoans on primary tierers and plants or by living in the burrows of primary tierers. Different primary tierers from soft substrata, nonreef, shallow subtidal shelf and epicontinental sea settings have had different tiering histories, resulting largely from contrasting constructional and phylogenetic constraints. Primary colonial tierers generally occupied lower epifaunal tiers during the Paleozoic and Mesozoic, but since the Cretaceous they have been dominant in the highest tier (+ 20 to +50 cm). Primary echinoderm tierers have been almost exclusively epifaunal, and from the Paleozoic through the Jurassic they were present throughout the epifaunal tiered structure. Although primary bivalve tierers have been both epifaunal and infaunal, they have occupied only lower epifaunal tiers, whereas they have adapted to all levels of the infaunal tiering structure, particularly from the late Paleozoic through the Recent. Brachiopods have lived primarily in tiers directly above or below the water-sediment interface and have not contributed significantly to tiering complexity. Of the numerous physical and biotic processes and constraints that affect shallow marine benthos, a few have contributed more significantly to changes in tiering patterns. Trends for increasing body size could have accounted for most of the development of tiering complexity up to +50 cm and down to –12 cm. Development of tiering above +50 cm could have been due to processes which would have yielded greater feeding capability, such as competitive interactions for a place from which to feed or adaptations to velocity gradients in the hydrodynamic boundary layer. The most significant process for development of infauanl tiering below –12 cm appears to have been as an adaptive response for predator avoidance. Unlike infaunal tiering, which never declined after it developed, epifaunal tiering has undergone a general reduction twice. Reduction in epifaunal tiering at the end of the Paleozoic appears to have been the result of the mass extinction at this time, whereas long-term biotic processes seem to have been more important for the tiering decline at the end of the Mesozoic. Tiering structure through the Phanerozoic was thus produced through interactions of a number of physical and biotic factors, tempered by constructional and phylogenetic constraints of each primary tierer group.

Paleoenvironmental patterns in the evolution of post-Paleozoic benthic marine invertebrates

The ecological context of large-scale evolutionary patterns has been neglected. Several workers have recently reported a bathymetric bias in the evolution of benthic marine communities, such that nearshore assemblages tend to contain advanced taxa and community structures and offshore assemblages contain more archaic features. A clade-by-clade analysis is the most powerful approach for assessing the generality of the pattern and testing hypotheses on underlying mechanisms. We develop a paleoenvironmental framework for constructing time-environment histories for higher taxa that should be of general comparative utility, based on five environmental categories recognized by physical sedimentary and biostratinomic criteria. The robustness of negative data (absence of a given taxon from a particular time and environment) is evaluated using taphonomic control groupshigher taxa with ecological, morphological, and mineralogical properties similar to those of the group under study. Within this framework we have constructed three detailed time-environment histories from the primary literature, for the crinoid Order Isocrinida (based on 99 early Triassic-Recent occurrences), the bryozoan Order Cheilostomata (67 late Jurassic-late Eocene occurrences), and the bivalve Superfamily Tellinacea (70 late Triassic-Miocene occurrences). These three time-environment diagrams as well as numerous anecdotal reports suggest that the onshore-offshore trends previously reported for communities are actually underlain by individualistic clade histories that only appear to act in concert when viewed on a coarse time scale. For these three particular higher taxa differences in rates and timing of movement through environments also falsify causal hypotheses that invoke sea-level fluctuations or mass extinctions. We outline remaining viable hypotheses for driving mechanisms and suggest further tests; at present the data are consistent with a broad array of intrinsic biological mechanisms. These pervasive shifts through time of environmental preferences severely undermine approaches to paleoenvironmental reconstruction based on interpreting fossil content in the light of present-day faunal distributions. Production of detailed timeenvironment histories for additional higher taxa will permit paleoenvironmental analyses based on overlapping environmental range zones, in which particular co-occurrences are diagnostic of different habitats at different times.

Restriction of a Late Neoproterozoic Biotope: Suspect-Microbial Structures and Trace Fossils at the Vendian-Cambrian Transition

Wrinkle structures are a class of oddly textured sedimentary structures that are common in Proterozoic-Cambrian marine siliciclastic strata, but uncommon in post-Ordovician subtidal marine facies. Despite a long history of study, there is little agreement on how these structures form, or their role in larger-scale sedimentologic and paleontologic contexts. Based on similarities with morphologic and sedimentologic characteristics of modern microbially dominated communities, it appears that ancient wrinkle structures could have been formed by microbial mats. Microbial genesis for wrinkle structures not only helps explain their ubiquity during the late Neoproterozoic, but also their distinct paleoenvironmental and temporal shifts related to increasing vertically-oriented metazoan bioturbation. The Vendian-Cambrian transition offers a unique opportunity to examine the influence of suspect-microbial communities on the siliciclastic sedimentary record and their relationship with the earliest burrowing, grazing, and locomotive activities of metazoans. Based on outcrops in the Great Basin, USA, metazoans appear to have been moving on, in, and under suspect microbially-bound sediment, and exhibit features that suggest active and passive epifaunal and infaunal sediment ingestion. Together with more intricate horizontal burrow networks, this style of horizontal bioturbation is common in shallow settings of the Vendian-Cambrian transition, and is hypothesized to reflect highly specialized approaches to exploiting mat-mediated organic-rich sediment layers. Post-Ordovician restriction of such bedding-parallel burrowing behavior to deeper settings mirrors a shift of suspect-microbial structures to stressed or deep-sea settings. This restriction suggests replacement (rather than progressive evolution) of metazoans with horizontally specialized ecological strategies by more vertically-oriented bioturbating organisms.